The present invention relates to a simple, high-performance brake fluid pressure control device mounted on an automobile for carrying out antilock brake control, traction control and automatic braking for controlling the stability of running vehicles.
Of various conventional antilock brake systems (ABS), what is known as fluid circulation type ABS's are relatively inexpensive and thus most widely used. An ABS of this type comprises on-off valves provided in each main fluid line connecting the master cylinder to the vehicle wheel brakes at positions near the respective wheels, a dumped fluid reservoir for temporarily storing brake fluid dumped from the wheel brakes, an on-off valve provided in a fluid line leading from the wheel brakes to the dumped fluid reservoir, and a pump for returning brake fluid in the dumped reservoir chamber back into the main fluid line. Brake fluid dumped from the wheel brake being pressure-reduced for antilock control is temporarily stored in the dumped fluid reservoir and returned by the pump into the main fluid line to repressurize the fluid in the wheel brake cylinder during antilock braking control.
If the fluid circulation pump of this device is used as a pressure source for traction control (herein after called T/C control), it is possible to control the vehicle behavior in a highly sophisticated and economical manner. For this purpose, unexamined Japanese patent publication 64-74153 proposes to add on-off valves to a fluid circulation type antilock device. The on-off valves connect the fluid supply lines to the main fluid lines during T/C control, and otherwise disconnect them. This arrangement has a problem in that the pump suction resistance is large because fluid is supplied from the master cylinder reservoir through a pipe line to the pumps during T/C control. The pumps thus can not dump brake fluid sufficiently quickly, which will impair responsiveness of the braking system.
Unexamined Japanese patent publication 6-72301 proposes to provide second reservoirs besides the dumped fluid reservoirs in order to more smoothly supply fluid to the pumps. But in this arrangement, if the pumps are activated while the fluid pressure produced in the master cylinder (hereinafter master cylinder pressure) is zero, the pistons in the second reservoirs will move, making it impossible to supply fluid during T/C control. Also, while the master cylinder pressure is being produced, the pistons for restricting the movement of the second reservoirs will go forward and back every time the brake pedal is depressed, thus extending the pedal stroke.
Unexamined Japanese patent publication 4-231241 proposes to use the active pressure accumulators with driving units as the second reservoirs. In this arrangement, there will be no unexpected behavior of the accumulators and the driving units make it possible to efficiently supply brake fluid to the pumps. But the accumulators and the driving units will increase the cost of the entire system.
The applicant of this invention proposed a brake fluid pressure control device that is free of all of the abovementioned problems and the problems of unexamined Japanese patent publication 5-116607, in unexamined Japanese patent publication 6-246400.
This device, shown in FIG. 3, comprises two main fluid lines 29 and 30 connecting a master cylinder 31 to wheel brakes 21-24 (of which 22 and 23 are driving front wheel brakes and 21 and 24 are non-driving rear wheel brakes); pressure control valves (consisting of on-off valve 7-10 for opening and closing the main fluid lines, and on-off valves 11-14 for opening and closing discharge lines) in the main lines 29, 30, dumped fluid reservoirs 17, 18 for temporarily storing brake fluid dumped from the wheel brakes through the on-off valves 11, 13 and 12, 14; pumps 19, 20 for pumping brake fluid in the dumped fluid reservoirs into the main fluid lines; fluid supply lines 32, 33 branching from the main fluid lines 29, 30 at points between the master cylinder and the fluid return points A1, A2 at which brake fluid from the pump is returned to the main fluid lines and leading to the dumped fluid reservoirs 17, 18; first changeover valves 1, 2 for checking fluid flow from the brake fluid return points A1, A2 toward the master cylinder 31 during traction control; relief valves 5, 6 for releasing excess pump dumped fluid into the main fluid lines while the first changeover valves 1, 2 are closed; and shutoff valves 3, 4 for checking fluid flow through the fluid supply lines 32, 33 toward the dumped fluid reservoirs 17, 18 while the master cylinder is being pressurized.
The improvement of this device lies in that each of the shutoff valves 3, 4 comprises an atmospheric chamber a, a fluid chamber b communicating with the dumped fluid chamber 18 (or 17), a piston p partitioning the chambers a and b, and a valve body v movable together with the piston p for shutting off the supply line 32 (or 33) from the dumped fluid reservoir 18 (or 17) when the piston p has moved toward the atmospheric chamber to its lower stroke end and otherwise permitting fluid communication therebetween. When the piston p moves from the lower stroke end toward the fluid chamber b to its upper stroke end, the volume of the fluid chamber b decreases by an amount at least equal to the amount of brake fluid supplied to the wheel brakes 21-24 from the pump 19 (or 20).
In this device, during T/C control (at which time the brake pedal is not depressed), the piston p is pulled by the pump 19 (or 20), so that brake fluid in the fluid chamber b is supplied to the pump 19 (or 20). While the fluid pressure is re-increasing during antilock control, brake fluid accumulated in the dumped fluid reservoirs while the pressure is decreasing during antilock control is drawn into the pumps 19 and 20. Thus, brake fluid is smoothly supplied to the pumps 19, 20 at all times. Since the shutoff valves 3, 4 serve as intermediate fluid reservoirs proposed in unexamined Japanese patent publication 5-116607, there is no need to provide separate intermediate reservoirs. It is thus possible to provide a smaller, less expensive fluid pressure control device.
Also, there is no need to provide valve means between the fluid chambers b as fluid supply chambers and the pumps 19, 20, so that the pumps can draw brake fluid without encountering any significant suction resistance.
Thus, the brake fluid pressure control device shown in FIG. 3, which the applicant of this invention proposed in unexamined Japanese patent publication 6-246400, has practically nothing to complain about as an ABS+T/C control device.
But this device can not satisfactorily carry out other types of automatic brake control such as for maintaining the stability of the running vehicle or for avoiding an rear-end collision.
For example, suppose the device shown in FIG. 3 has the vehicle stability control function. If the driver turns the steering wheel counterclockwise in an attempt to change lanes as shown in FIG. 4 while the vehicle is traveling at high speed, then centrifugal force will act on the vehicle which tends to skid the rear of the vehicle in the direction of the arrow Q. The brake fluid control device thus detects this force Q through e.g. an inertia sensor, and applies a braking force only to the front right wheel FWR to counteract the force Q. But if the brake pedal is being depressed to such an extent that no ABS control is initiated when the force Q is detected, the device of FIG. 3 cannot increase the brake pressure on the front right wheel (the wheel brake 22 in FIG. 3 corresponds to the front right wheel FWR in FIG. 4, 23 to FWL, 21 to RWR and 24 to RWL), because in this state, the piston p of the shutoff valve 4 is unable to move under the master cylinder pressure and thus the pump 19 cannot draw brake fluid necessary to increase the brake pressure on the wheel FWR from the fluid chamber b. That is, the pressure control device cannot perform vehicle vehicle stability control in this state.
It is also feasible to provide the device of FIG. 3 with an anti-collision means which measures the distance from a front obstacle (such as a car) and automatically applies the brakes if the above distance is dangerously small. Suppose the car equipped with this means moves dangerously close to an obstacle and the driver depresses the brake pedal in an attempt to avoid collision. If, in this state, the electronic control unit of the pressure control device judges that the braking force is still insufficient based on sensor information, the control unit will endeavor to automatically increase the braking force by activating the pumps. But in this state, the pistons of the shutoff valves 3, 4 will not move under the master cylinder pressure, so that no fluid can be supplied to the pumps 19, 20, and thus no automatic braking is possible.
An object of this invention is to provide a brake fluid pressure control device capable of performing not only ABS control and T/C control, but other automatic braking control operations such as vehicle stability control and anti-collision control.